High speed optical scanning systems, such as precision plotters, printers and the like, are well known in the art. These devices are often used to fabricate printed circuit board (PCB) artwork by raster scanning onto film. A typical imaging system, such as is marketed by the Gerber Scientific Instrument Company, consists of a magnetic tape drive, hard disk, computer interactive graphics terminal, image processor, optical table having a moveable write platen for positioning the substrate and a precision laser scanner. The system also includes such other optics, media carriage and electronics as is necessary to directly transfer computer aided design (CAD) data to PCB artwork.
In operation, the direct imaging system is configured to receive on the write platen a planar substrate of film or glass which has an optically sensitive media on a surface. The computer modulates the intensity of an optical beam, usually provided by laser, to expose selected portions of the substrate. Typically, there is a second reference beam nearly colinear with the exposing beam for accurately controlling the position of the exposing or "write" beam on the substrate. A flat-field scanning system is employed to focus the beams to a small spot and to scan the beam simultaneously across a reference mask and the substrate. Precision air bearings are often used to guide the write platen as it is imaged.
Accuracies for these machines are typically plus or minus 0.001 inch or less for the entire field, and plus or minus 0.0002 inch or less for close features. The image traces are often as fine as 0.002 inches. With such extreme tolerances, a major problem has been to establish the dimensional accuracy of the machine. However, calibration of the imaging system alone does not guarantee that the artwork used in the printed circuit board fabrication process has that dimensional accuracy. Each image on the film substrate must be processed to develop the artwork image. During processing, which includes immersion in a variety of chemicals, the film substrate will expand and contract. Existing techniques for determining the dimensional accuracy of PCB artwork produced on a substrate require estimating the amount of shrinkage or expansion which has occurred during processing and extrapolating the accuracy of the artwork from that estimate.
It would be advantageous to have a method and apparatus for calibrating artwork produced by a direct imaging system which would determine produced image metrology--both the accuracy of the imaging system and loss of accuracy due to the film. The present invention is directed towards such a method and apparatus.